When users confirm Duo MFA in the mobile app, live-update the UI

Summary: Ref T13249. Poll for Duo updates in the background so we can automatically update the UI when the user clicks the mobile phone app button.

Test Plan: Hit a Duo gate, clicked "Approve" in the mobile app, saw the UI update immediately.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13249

Differential Revision: https://secure.phabricator.com/D20169